This invention relates to an industrial manufacturing method for alklydihalogenophosphines, which are industrially useful as intermediates in the manufacture of pesticides, flame retardants, and other organic phosphorus compounds.
Alkyldihalogenophosphines, and particularly the industrially useful compound methyldichlorophosphine, can be manufactured by the direct reaction of phosphorus trichloride and methane in the presence of carbon tetrachloride at a temperature of at least 500.degree. C. Manufacturing methods employing direct reaction are disclosed in Japanese Published Unexamined Pat. applications Nos. 53-5123, 53-18517, 53-23929, and 58-116494.
On the other hand, the Friedel-Crafts reaction between alkylchlorides and phosphorus trichloride is well known. Alkyldichlorophosphines can be manufactured by reacting the complex which is formed by the Friedel-Crafts reaction using red phosphorus, antimony, or the like as a reducing agent. See Zhur. Obshch. Khim (Vol. 28, pp. 2963-2965 (1958)) or Canadian Journal of Chemistry (Vol. 41, p. 2299 (1963)).
The above-described manufacturing method for methyldichlorophosphine in which phosphorus trichloride and methane are directly reacted is important because it is an extremely rational method, but since it is a vapor phase reaction carried out at a high temperature of at least 500.degree. C., the materials forming the reaction apparatus must be carefully selected, and as the raw materials are chlorides, special precautions must be taken to prevent environmental pollution. In addition, not only is the reaction dangerous, but it produces many by-products, and the separation of the desired product from the by-products is extremely difficult.
On the other hand, the method in which the complex which is formed as the reaction product of the Friedel-Crafts reaction between alkylchloride and phosphorus trichloride is then reduced is a solid-solid, solid-liquid or liquid-liquid reaction, so treatment is easy compared to the treatment required for the direct reaction method. However, even if the complex which is formed as a reaction product is reduced with a reducing agent such as red phosphorus, subsequent processes are necessary to obtain the desired product. In the subsequent processes, a large amount of aluminum chloride (AlCl.sub.3) is formed, and the separation and treatment thereof are difficult, so it is difficult to recover high-purity alkyldichlorophosphine with a high yield. For example, if red phosphorus is used as a reducing agent, as the reaction is carried out at a high temperature without a solvent, it is difficult to perform the reaction industrially. If antimonY is used as a reducing agent, the inclusion of impurities is unavoidable.
There is also a method in which the complex (Cl.sub.3 C-PCl.sub.3 .multidot.AlCl.sub.4) which is formed as a reaction product of the Friedel-Crafts reaction is reduced using yellow phosphorus as a reducing agent (U.S. Pat. No. 2875224). However, not only is the reaction inadequate, but that patent does not teach a manufacturing method for alkyldihalogenophosphines.